The present invention relates generally to the creation of automotive engine valves using a powder metallurgy process, and more particularly to intake and exhaust valves, where at least portions of each are made by one or more of such processes.
Improved fuel efficiency is an important goal in automotive design. One way to achieve this is through the use of lightweight materials and components. Traditionally, rapidly moving and reciprocating parts, such as engine intake and exhaust valves, have been made from refractory materials, such as steels, superalloys or the like. Such materials, while robust enough to endure the rigors of the internal combustion process, tend to be heavy. This additional weight has an ancillary impact on other components, such as springs, rocker arms, bearings or the like that cooperate with and must therefore be able to withstand the extra forces imposed by the valves.
The introduction of titanium has allowed designers to rely less on refractory materials, providing much of the structural and temperature requirements at a fraction of the weight of steels, superalloys and related refractory materials. Precise additions of alloying ingredients, such as aluminum, vanadium or the like can be used to tailor the structural properties of titanium. For example, the fatigue strength at high temperature for exhaust valve stems must be high, yet not so much so that cold workability and related manufacturing is hampered. Likewise, such agents used in the head portion of an intake valve enhance the strength and hardness; where the tradeoff between wear resistance and component embrittlement must be balanced.
Despite these advantages, titanium has not enjoyed widespread use in engine valve applications. One significant drawback to titanium is that it is expensive to manufacture, especially in light of the differing environmental conditions and requirements at various locations within the valve, such as the valve tip, stem and head. For example, the valve head is subjected to a high temperature environment (up to 1400° Fahrenheit) over significant durations, which could lead to significant creep loading. Likewise, the valve stem temperatures are a little lower (up to 1200° Fahrenheit), but are subjected to significant camshaft and valve spring forces, where compression, tension, shock and fatigue strength properties become important. These concerns are especially relevant to the remote tip region of the valve stem.
Traditionally, engine valves have been made by forging (particularly upset forging) followed by heat treatment and machining, where a titanium alloy rod material is manufactured from an ingot of titanium alloy, that is then molded then hot swaged so as to form a valve shape. Such approaches are labor-intensive, as well as wasteful of the material. Casting techniques have also been used; however, mechanical properties have been less than with forging, and are also not well-suited to using disparate materials within a single casting. More sophisticated casting techniques, such as local chilling or controlled microstructure variation through localized aging can improve the casting, but do so at increased cost, and are often limited to certain (specifically, ferrous-based) materials. With conventional powder metallurgy, a metal alloy powder is compacted to a molded valve shape by cold isostatic pressing, and then sintered. Residual pores in the as-sintered body results in lower ductility and fatigue strength.
Thus, it is desirable that an improved method of making high strength, titanium-based components, such as engine valves, be developed. It is further desirable that different approaches best tailored to particular parts of an engine valve be used to manufacture the valve. It is further desirable that an engine valve made by such method be reliable enough for longer-term use. It is further desirable that a low-cost powder metallurgy manufacturing process that minimizes the likelihood of residual porosity formation be used to make at least portions of such a valve.